This invention relates generally to the field of chlorosilane production and more specifically to a process for removing aluminum and other metal chlorides from chlorosilanes.
In most processes for production of high purity silicon, which is in increasing demand for photo-voltaic devices, the first step is to convert impure, approximately 99% silicon, known as metallurgical grade silicon, MGS, into a liquid chlorosilane, usually trichlorosilane, SiHCl3, which can be purified to very high levels and then converted back to very high purity solid silicon. In this first step, which typically takes place in a fluidized bed reactor, the impure solid silicon is reacted with a chlorine containing gas and many impurities are retained as solids in this reactor or in dust removal equipment such as cyclones. However, the properties of aluminum, and some other contaminants, such as antimony, boron, carbon, indium, gallium phosphorus, thallium, tin, titanium, zinc and zirconium, are such that they form volatile compounds which are carried out of the reactor with the desired chlorosilane. Thus they are present in the effluent gas from the reactor which is then cooled to form a liquid chlorosilane mixture, whose main ingredients are dichlorosilane, SiH2Cl2, trichlorosilane, SiHCl3, and silicon tetrachloride (also known as tetrachlorosilane), SiCl4, which can be purified by conventional means, primarily distillation. Aluminum is particularly important because it is present in large quantities, 2000-10000 ppma in the metallurgical grade feed stock and, like boron, acts as an electrically active dopant in high purity silicon and so must be reduced to very low levels; boron, however, is only present in the metallurgical grade feed stock at about 20-100 ppma. Furthermore, aluminum chloride, AlCl3, has unusual properties in that it does not form a liquid phase at atmospheric pressure. At close to atmospheric pressures such as would typically be used for distillation, it converts directly from a solid to a gas; it is, however, partially soluble in chlorosilanes dependent on temperature. Thus it is possible to remove aluminum chloride by distillation but very difficult as it tends to form solid deposits within the distillation system and it makes it impossible to directly generate a liquid waste with a high concentration of aluminum, thus requiring disposal of more waste with high economic and environmental impacts. As noted above there are other metals which also form volatile compounds and which are also chlorides. Of these chlorides, most, antimony, indium, gallium, thallium, tin, zinc and zirconium, behave similarly to aluminum and thus tend to be removed with it and one, titanium, does not. Of the remaining elements, boron, carbon and phosphorus, which form volatile compounds, the boron and carbon compounds do not behave like aluminum and must be removed in some other way. The phosphorus compounds also do not behave like aluminum, but certain phosphorus compounds, PH3, PH4Cl, PCl5 and POCl3, can bind with aluminum chloride to form adducts and be removed with the aluminum, and one, PCl3, does not. Adducts are weakly bound mixtures of a Lewis acid and Lewis base and so can form and dissociate readily. This capability of the solid aluminum chloride/phosphine adducts to dissociate is particularly of concern because solids trapped in filters or tanks may release gaseous or dissolved phosphine unexpectedly and cause a spike in phosphorus concentration.
Most prior art patents in chlorosilane production do not mention removing metal chlorides nor do they mention removing phosphorus by binding it to aluminum chloride. In U.S. Pat. No. 4,676,967 by Breneman “High Purity Silane and Silicon Production” the presence of metal chlorides are mentioned as being removed incidentally as part of a waste stream whose primary purpose is the removal of carryover metallurgical silicon powder. Solids are allowed to settle in the bottom of the column and the bottom contents of liquid and solids are periodically blown down to disposal. This is the “only waste stream of the overall integrated process.” (Page 5 line 40)
US Patent Application US 2004/0042949 A1 by Block et al. “Method for Removing Aluminum from Chlorosilanes” and Block et al U.S. Pat. No. 6,887,448 “Method of Production of High Purity Silicon”
These inventions use distillation at a temperature greater than 160° C. and high pressure (25-40 bar).
US Patent Application 2007/0098612 A1 by Lord “A Set of Processes for Removing Impurities from a Silicon production Facility”
This application discusses various prior art processes and mentions in passing that a difference between chlorosilane and bromosilane based processes is that in the chlorosilane based process, an additional filtration step is required to remove the solid aluminum chloride.
The deficiencies of the prior art separation technology is also discussed in the prior art technology for processing the wastes that contain the aluminum.
In Ruff, U.S. Pat. No. 5,066,472 page 1 line 28 “The chlorosilanes are usually roughly separated from the solid residues by distillation, leaving as residue a suspension that requires separate processing.” He further states on page 1 line 67 “The problem therefore exists of finding a method for processing the distillation residues with the recovery of chlorosilanes . . . . ”
As a first step the residue is concentrated by evaporation in a screw dryer.
Similar steps are taken in Breneman U.S. Pat. No. 4,743,344 and in the Breneman patent application US 2006/0183958.
Thus it is clear that a primary deficiency of the prior technology is that the waste stream containing the aluminum contains too much valuable chlorosilanes and considerable energy must be expended to recover this material.
Block, US 2004/0042949 A1, reveals a further deficiency of the prior art distillation separation which is that the aluminum chloride spreads throughout the whole column by sublimation in the gas phase leading to failure to separate the aluminum and to deposition of solid aluminum chloride throughout the column and ultimately to shutdown of the column for cleaning. His invention of high temperature and high pressure (25-40 bar) distillation keeps the aluminum chloride liquid but also has a similar drawback of high energy consumption and high capital cost because of the high pressure. The energy consumption is known to be high because virtually the entire effluent from the reactor is boiled off overhead. Similarly, the capital cost is high because the entire plant effluent must be distilled. A further distillation is still required to separate the desired trichlorosilane from the byproduct silicon tetrachloride.
Lord, US 2007/0098612 A1, does not identify either how to filter the aluminum chloride or, more importantly, how to cause the formation of suitable solids that may be easily filtered. A filtration process also suffers by being a batch process with high capital cost.
Further deficiencies in the prior technology are that there is no mention of the fact that the metal chlorides are less soluble in trichlorosilane than in silicon tetrachloride or that trapped solids containing aluminum chloride may adsorb and release phosphine, PH3, or the other possible phosphorus compounds, PH4Cl, POCl, PCl5, which bind to aluminum chloride.